Thanks, @ashthespy!
Got the latest Volumio Debian Buster Beta up and running, and audio over HDMI is functional. However, I noticed any USB devices plugged into the Pi 4 USB 3.0 or 2.0 ports are not detected by the OS. This includes my Logitech USB dongle for wireless keyboard and my KEF X300A speakers. Volumio thus fails to list the X300A as a playback option.
I’m keeping my fingers crossed that Debian Buster will eventually catch up to supporting the VL805 USB 3.0 host controller on the Pi 4.